In general, a light emitting diode (LED) is widely used as a light source because it has characteristics of small size, low power consumption, and high reliability. A compound semiconductor such as InGaSaP, AlGaAs, GaAlP, GaP, InGaAlP or GaN is used for the LED. The LED includes an N-type semiconductor layer made of a compound semiconductor, an active layer on the N-type semiconductor layer, and a P-type semiconductor layer on the active layer. The LED is a kind of p-n junction diode and is a semiconductor device using electroluminescence, in which light is emitted when forward voltage is applied to the semiconductor device. The center wavelength of the light emitted from the LED is determined by bandgap energy (Eg) of a semiconductor used for the LED.
Temperature dependent electroluminescence (TDEL) method is most commonly used as a method for measuring internal quantum efficiency of the LED at a specific temperature (e.g., room temperature). In the method, it is assumed that the internal quantum efficiency (ηIQE) is 100% under a condition that the relative radiative efficiency (η) (i.e., η=P/I) defined as a ratio of the intensity (P) of light emitted from the LED to injection current (I) at an extremely low temperature (about 10K or less) is maximized, i.e., in the maximum injection current (Imax) having the maximum relative radiative efficiency (ηmax) (i.e., ηmax=Pmax/Imax). The internal quantum efficiency (ηIQE) in predetermined injection current (I) at a specific temperature (e.g., room temperature) is obtained from a ratio of the relative radiative efficiency (η=P/I) under the same condition to the maximum relative radiative efficiency (ηmax=Pmax/Imax) at an extremely low temperature, i.e., (P/I)/(Pmax/Imax). However, the case where it can be assumed that the internal quantum efficiency is 100% as the temperature becomes extremely lower is limited to the case where the maximum value of the relative radiative efficiency gradually increases to a specific maximum value as the temperature becomes lower. Also, it takes a very long time (about 5˜6 hours) to change the temperature from an extremely low temperature to a room temperature, and a high-priced device for temperature tests is required. Since an extremely small portion of a wafer is to be cut and measured due to restriction of the size of a chamber in the device for temperature tests, the internal quantum efficiency of the entire wafer cannot be measured. The external quantum efficiency (ηEQE) defined as (number of photons coming out into free space per unit time)/(number of electrons injected in optical element) can be experimentally measured. The external quantum efficiency (ηEQE) is defined as a multiplication of internal quantum efficiency (ηIQE) and light extraction efficiency (ηextraction). Hence, if the internal quantum efficiency is to be measured, the internal quantum efficiency and the light extraction efficiency can be separately measured.